The field of the present invention is computed tomography and, particularly, computer tomography (CT) scanners used to produce medical images from x-ray attenuation measurements.
As shown in FIG. 1, a CT scanner used to produce images of the human anatomy has a patient table 10 which can be positioned within the aperture 11 of a gantry 12. A source of highly collimated x-rays 13 is mounted within the gantry 12 to one side of its aperture 11, and one or more detectors 14 are mounted to the other side of the aperture. The x-ray source 13 and detectors 14 are revolved about the aperture 11 during a scan of the patient to obtain x-ray attenuation measurements from many different angles through a range of at least 180.degree. of revolution.
A complete scan of the patient is comprised of a set of x-ray attenuation measurements which are made at discrete angular orientations of the x-ray source 13 and detector 14. Each such set of measurments is referred to in the art as a "view" and the results of each such set of measurements is a transmission profile. As shown in FIG. 2A, the set of measurements in each view may be obtained by simultaneously translating the x-ray source 13 and detector 14 across the acquisition field of view, as indicated by arrows 15. As the devices 13 and 14 are translated, a series of x-ray attenuation measurements are made through the patient and the resulting set of data provides a transmission profile at one angular orientation. The angular orientation of the devices 13 and 14 is then changed (for example, 1.degree.) and another view is acquired. An alternative structure for acquiring each transmission profile is shown in FIG. 2B. In this construction, the x-ray source 13 produces a fan-shaped beam which passes through the patient and impinges on an array of detectors 14. Each detector 14 in this array produces a separate attenuation signal and the signals from all the detectors 14 are separately acquired to produce the transmission profile for the indicated angular orientation. As in the first structure, the x-ray source 13 and detector array 14 are then revolved to a different angular orientation and the next transmission profile is acquired.
As the data is acquired for each transmission profile, the signals are filtered, corrected, converted to logarithmic form and digitized for storage in a computer memory. These steps are referred to in the art collectively as "preprocessing" and they may be performed in real time as the data is being acquired. The acquired transmission profiles are then used to reconstruct an image which indicates the x-ray attenuation coefficient of each voxel in the reconstruction field of view. These attenuation coefficients are converted to integers called "CT numbers," which are used to control the brightness of a corresponding pixel on a CRT display. As illustrated in FIG. 3, an image which reveals the anatomical structures in a slice 15 taken through the patient is thus produced.
In clinical applications the thickness of the slice 15 taken through the patient may be varied from very thin (1 mm) to very thick (10 mm). The slice thickness is typically controlled by an adjustable collimation device which is positioned between the patient and the x-ray source. One such collimation device is described in U.S. Pat. No. 4,991,189 which is owned by the assignee of the present invention.
As the thickness of the slice 15 is increased, the reconstructed image becomes more susceptible to partial volume artifacts. The CT number at each image pixel represents the attenuation of the x-ray beam by the corresponding voxel in the patient. For infinitesimal thin beams, an accurate measurement of the integral attenuation along the x-ray beam could be made, if sufficient flux can be detected, so that the CT number does reflect a true average attenuation of all material in the corresponding patient voxel. However, for x-ray beams having a finite thickness, and where the attenuation of the material is inhomogeneous in the thickness direction, an accurate measurement of the average attenuation across the beam is not achieved. This inaccuracy is pronounced, for example, in patient voxels which contain a boundary between highly attenuating material such as bone and soft tissues. Because of the nature of the image reconstruction process, this inaccuracy not only affects the corresponding image pixel, but also surrounding pixels. This results in image artifacts which interfere with the diagnosis of soft tissue features.
There are two known techniques for dealing with this partial volume artifact problem when imaging thick slices. First, attempts have been made to predict the distribution of bone in a slice using data acquired from neighboring slices. Such predictions can be used to correct the data, however, in practice the inter slice spacing between slices is too large to provide accurate predictions. The second approach is to acquire the thick slice as a series of separate, but contiguous thin slices. As the slice is made thinner, the flux of x-rays intercepted by the detector is reduced. This results in a noisier image, and for large patients, the detected signal may fall below the noise level of the detector and no image can be reconstructed. In this case it is necessary to increase the x-ray tube flux, or increase the integral time for the detected signal. While this works, the time required for data acquisition and data processing is multiplied by the number of thin slices required. In addition, x-ray tube cooling rate and heat capacity may further lengthen the scan time, and patient movement during the longer scan may corrupt the acquired data. As a result, it is not practical to scan larger volumes with thin slices because of the long time involved in scanning the patient, reconstructing the images and waiting for the x-ray tube to cool, and the higher x-ray dose to the patient.